Method and apparatus for modifying limit and protection software in a synchronous generator exciter to match the capability of the turbine-generator

ABSTRACT

A method and apparatus for compensating, consistent with variations in cooling conditions, protection and limit functions of a generator. Limits of the capability curve of the generator, including the overexcited region, underexcited region and region limited by stator current heating, are modified in response to changes in coolant pressure or temperature depending on the type of generator being compensated. Feedback signals and setpoints being provided to fixed protection and limit functions are intercepted and modified in accordance with the relationship between generator rated values and generator performance in view of altered coolant conditions. Protection and limit functions are automatically coordinated. As a result, overall generator performance is improved.

BACKGROUND

[0001] Exciters designed for operation with synchronous generators havetraditionally been required to protect the generator field by limitingoperation of the generator in the overexcited region of a generator'scapability curve which is restricted by field heating. In theunderexcited region of the generator capability curve, operation must belimited because of stator end turn heating and stator laminationover-voltage effects. Operation between rated KVA at rated lagging powerfactor and rated KVA at unity power factor must be limited because ofoverheating caused by excessive stator current. While conventional fixedhardware or software algorithms used to implement this functionalitysuch as General Electric's GENERREX-CPS, GENERREX-PPS, SHUNT-SCR,ALTERREX, STATIC BUS FED EXCITATION and SILCO 5 control implementations,can properly protect the generator these prior art implementations donot have any capability of being responsive to coolant conditions asthey deteriorate from nominal. A further problem is that prior artlimiter implementations do not account for improved coolant conditionsand the possible increase in generator rating associated with suchimproved coolant conditions. Indeed, “ambient following” combustionturbine applications are particularly sensitive to changes in coolantconditions as the output power capability thereof varies significantlywith coolant conditions.

[0002] It is thus seen to be desirable to coordinate the limit andprotection functions of a generator exciter as a function of coolantconditions to provide improved overall generator performance.

SUMMARY OF THE PREFERRED EMBODIMENTS

[0003] The apparatus and method described herein accurately protect thegenerator as coolant conditions vary from nominal in the areas limitedby (1) field heating, (2) stator current heating and (3) underexcitedcapability while maintaining full output capability from the generator.In a preferred embodiment, appropriate algorithms implemented inhardware or software are automatically selected to optimize thecompensation for either hydrogen cooled or air cooled generators in thearea of the capability curve characterized by field current or statorcurrent heating while a jumper, or other suitable switching device, isimplemented to select the proper compensation in the underexcited area.The advantages of the preferred embodiment include implementation ofvery accurate compensation of limiters with automatic coordination ofthe limiters and protection algorithms, whereby a more efficient use ofa generator across varying cooling conditions is realized.

[0004] Since both limit and protection algorithms can be supplied in anexciter of a generator, it is, in accordance with the preferredembodiment, desirable that the limit and protection functions becoordinated and maintain coordination as coolant conditions change.Thus, described herein, is a very accurate apparatus and method forcoordination of the limit and protection functions, as a function ofcoolant conditions, permitting full output capability from a turbinegenerator while maintaining very accurate protection functions. As anadded benefit, a coolant compensated stator current limit function canbe added for operation in the area of the capability curve from ratedKVA at rated lagging power factor to operation at rated KVA and unitypower factor.

[0005] More specifically, generators have different ratings based on thecoolant conditions at the existing operating point. As noted above,prior art exciter implementations use fixed limit and protectionhardware or software algorithms which either do not offer protection atother than rated coolant conditions or do not permit generator operationto increase as coolant conditions improve to obtain full capabilityoperation of the generator. This is especially important for ambientfollowing combustion turbine applications, as previously explained.Thus, the methodology described herein implements fixed parameter limitsand protection algorithms yet provides full limit and protectioncompensation keyed either to temperature or hydrogen pressure (dependingon the type of cooling system) to match the capability of the exciter tothe generator rating. And, as a consequence, there is provided automaticcoordination between the limit and protection algorithms.

[0006] In accordance with the preferred embodiment, a two partcharacteristic is used to implement the under excited limit (UEL).Utilizing such a two part characteristic for the under excited limit hasthe added benefit of providing a compensating signal for operation inthe area of the generator capability curve determined by stator currentheating or stator KVA. This permits the use of a fixed parameter statorcurrent limit that is compensated for coolant conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 illustrates a typical air-cooled generator capabilitycurve.

[0008]FIG. 2 illustrates a typical hydrogen-cooled generator capabilitycurve.

[0009]FIG. 3 illustrates a typical European manufacturer's air-cooledgenerator capability curve.

[0010]FIG. 4 illustrates a block diagram showing fixed limiters andprotection with coolant compensation in accordance with a preferredembodiment.

[0011]FIG. 5 illustrates a hydrogen pressure and ambient temperatureinput sensor in accordance with a preferred embodiment.

[0012]FIG. 6 illustrates an underexcited limiting and stator currentlimiter arrangement for compensation of hydrogen pressure and ambienttemperature in accordance with a preferred embodiment.

DETAILED DESCRIPTION

[0013]FIG. 1 illustrates a typical reactive capability curve for anair-cooled generator. The generator rating at rated inlet airtemperature is given by the curve through points A B C and D. Two othercapability curves are shown in FIG. 1, demonstrating the capability ofthe generator when operating at temperatures that differ from rated. Thecurve from A to B is limited by field heating in the generator andresults in a maximum field current value that is permitted for steadystate operation. The over excited limit (OEL) and associated protectionis used to limit operation of the generator in this section of thecapability curve. Curve B C is limited by stator current heating. Astator current limit is used to restrict operation of the generator whenthe generator output falls within this region of the capability curve.Curve C D reflects the capability of the generator to operate in an areaof the capability curve limited by end iron heating and statorlamination over-voltage. Curves E F G D and H J K D illustrate thegenerator capability at 50 Degrees C. and −18 degrees C. respectively.

[0014]FIG. 2 illustrates a typical hydrogen-cooled generator capabilitycurve. In the case of hydrogen-cooled generators, generator ratingoccurs when the generator is at the maximum hydrogen pressure and thegenerator rating decreases as hydrogen pressure is reduced. The ratedhydrogen pressure capability curve is denoted as curve ABCD in FIG. 2and the curve for the lowest hydrogen pressure is denoted as curve EFGD.The capability curve limitations are similar to the curves in FIG. 1.That is, in FIG. 2 curve A B is limited by field heating, curve B C islimited by stator current heating, and end iron heating and statorlamination over-voltages limit curve C D.

[0015]FIG. 3 illustrates a typical air-cooled generator capability curvefrom a European manufacturer. Instead of the common US practice ofshowing the zero power axis on the left side of the graph, the standardEuropean practice, as illustrated in FIG. 3, is to show the zero powerpoint on the right. The capability curve limitations are similar to thecurves in FIG. 1. Curve A B is limited by field heating. Curve B C islimited by stator current heating. End iron heating and statorlamination over-voltages limit Curve C D. The curves from C to D, G to Dand K to D illustrate a difference between these generators and thestandard U.S. air-cooled generators. None of these curves shows anyvariation with coolant temperature.

[0016] In accordance with the preferred embodiments discussed herein,there is provided limiters and protection apparatus and methods(hereinafter “algorithms” or “functions”) for exciters for synchronousgenerators. Specifically, the limiters disclosed herein, throughregulator action, limit operation of the exciter and generator toacceptable operating areas of a generator's capability curve. Protectionalgorithms, on the other hand, preferably do not have any regulatorfunctions but instead generate trips in response to operation of theexciter and generator outside the capability curve of the generator. Asdescribed herein there is provided limiter algorithms that activelyforce the exciter to operate within the capability curve in steady stateconditions and can be compensated for coolant conditions.

[0017] Further, the preferred embodiment permits transient penetrationof a coolant compensated forbidden area of the capability curve forsystem events, thus maximizing the contribution of the generator tosystem stability. By implementing similar algorithms for the limiter andprotection functions it is possible to effectively coordinate betweenthe limiter and protection functions. Consequently, it is possible, inthe instance of the OEL, to ensure that the limiter, if it is working,will limit operation of the generator to the coolant compensated area ofthe generator capability curve limited by generator field heating. Thisensures that the protection does not trip undesirably, therebymaintaining the generator on-line for longer periods and during periodsduring which conventionally would result in a tripping situation. Inother words, the limiter algorithms control the amount of currentflowing in the field winding and does so in coordination with protectionalgorithms.

[0018] Specifically, in the field circuit of a generator, the totaltemperature of the field winding is an important criterion. The totalfield winding temperature is affected by the field current in thewinding and the condition and temperature of the coolant. Generally,generators used in steam turbine applications tend to operate at a fixedcold gas temperature and do not follow ambient temperature variations.In this instance, the gas pressure varies instead and the capability ofthe generator decreases with lower gas pressures. The relationshipbetween the generator field current and gas pressure in a gas cooledgenerator can be defined as a log-log function which is well known tothose skilled in the art. Specifically, if the present gas absolutepressure is (PGAS+14.7) lbs/in² (hereinafter units of pressure andtemperature, e.g., degrees C, are to be understood, although notexpressly stated) then the maximum allowable field current at thatpressure is

IF@PGAS=IF_RAT*((PGAS+14.7/(PGAS_RATED+14.7)){circumflex over ()}(H2FEXP/10000)   (1)

[0019] IF_RAT (rated field current) and the term H2FEXP can bedetermined from generator data for a particular application. IF@PGAS isthe field current rating specified by the manufacturer at a particulargas pressure. H2FEXP is an exponent that can be estimated from the fieldcurrent and hydrogen pressure data supplied by the generatormanufacturer and the “10000” divisor in Equation (1) results from thespecific implementation used to relate digital counts to engineeringunits. Generally, the term (H2FEXP/10000) varies between 0.4 and 0.5 fortypical hydrogen cooled generators. The preferred embodiment maximizesthe accuracy of the approximation illustrated in Equation (1) forraising the ratio of the two absolute temperature signals to a powerthat varies between 0.4 and 0.5.

[0020] In this regard, both sides of Equation (1) can be divided byIF_RAT which yields, on the right hand side of such a modified Equation(1), a multiplier which is used to modify (a) feedback signals and/or(b) limit reference setpoints. This permits implementation of protectionfunctions and limiters that are capable of being compensated for coolantchanges while still using fixed algorithms for the protection andlimiter functions.

[0021]FIG. 4 illustrates a block diagram of the preferred implementationof a coolant compensated under excited limit (UEL), OEL (both limiterand protection) and stator current limit. Only the blocks pertinent tothe implementation of coolant compensation are described in detail. Theblock labeled UELIN, block 400, is the input block for the UEL functionand provides the functions of normalizing by 1/VT{circumflex over ( )}2,low pass filtering, enabling and an absolute value circuit. Inputs powerP and terminal voltage V_(T) are provided to block 400. The table lookup, block 405, is an arbitrary function generator, used to implement thecapability curve from point C to D on the rated coolant capabilitycurve. The block labeled UELOP, block 415, is used to remove the1/VT{circumflex over ( )}2 normalization from the coolant compensatedVar reference, the output from the UELHT block, block 410. The blockE1PFV, block 425, uses the signal HTSMAXVA (explained below) to modifythe stator current limit reference so that raise and lower commands areissued to an automatic voltage regulator (AVR) (not shown) to limitstator current consistent with the coolant conditions. Ratio block 430is used to modify the OEL field current regulator reference. Ratio block435 is used to modify the field current feedback signal to the limit andprotection algorithms.

[0022]FIG. 5 illustrates a preferred technique to develop a multiplier(HTS_BIAS) to compensate the over excited limiter (OEL) and protectionalgorithms for hydrogen cooled generators. The term HTS@IN is the outputsignal of a hydrogen pressure transducer associated with the coolingsystem of the generator (not shown) and generally supplied by thegenerator manufacturer. This signal is passed through low pass filter500 to remove undesirable AC components. Further, the term HTSOFS(hydrogen temperature sensor offset) is selected to remove, via summingblock 515, any offset resulting from the use of, for example, a 4-20 macurrent signal for transmitting hydrogen pressure (as embodied inHTS@IN). In the case of hydrogen cooled generators, the current signalis indicative of coolant conditions, as discussed previously herein. Itis common practice to use an offset transducer (4-20 ma) so that a zerosignal level is not permitted and signal levels less than, for instance2 mA., can be interpreted as an open wire.

[0023] The term HTSSCL (hydrogen temperature scalar) in scaling block520 provides the ability to scale the absolute hydrogen pressure signalfor 10000 counts at rated absolute pressure. The use of the 10000-countdivisor is arbitrary and any suitable value can be used depending onimplementation considerations. The term HTSFAL (hydrogen temperaturesensor failure), fed to comparator 545, permits detection of a failurein the input signal (HTS@IN) from the transducer that results from,e.g., an open wire. This is used to generate a signal HTSENBAD (hydrogentemperature sensor bad) for diagnostic purposes (as desired) andpreferably to switch, via switch 530, the input signal to a value HTSNOM(hydrogen temperature sensor nominal) so that no compensation is appliedwhen the sensor or transducer fails. HTSSENSE provides a diagnosticsignal representative of the output of switch 530, Block 525 provides anupper limit (HTSULM) and lower limit (HTSLLM) for the value exitingscaling block 520.

[0024] TMPRAT (rated temperature) is entered as a zero in this instanceresulting in the automatic selection of the uppermost path, namelyfunction block 550, shown on the right hand side of FIG. 5. This resultsin the generation of HTS_BIAS (the multiplier) for compensating, as afunction of hydrogen absolute pressure, the OEL and protection. In thepreferred implementation, HTS_BIAS is scaled, in multiplier block 575,for 10000 counts equal to unity ratio. The preferred technique is to usea ratio block (FIG. 4) to multiply the feedback signal (IFG) to aninverse time protection algorithm by 10000/HTS_BIAS and the reference tothe OEL (FCR_REF) by HTS_BIAS/10000 (ratio blocks 435 and 430respectively) thereby providing compensated reference values to thefixed limit and protection algorithms.

[0025] In contrast to steam powered generator applications, generatorspowered by combustion turbines operate in an ambient following mode at afixed gas pressure (which would be zero for an air-cooled machine). Therelationship between field current and gas temperature is based onquadratic interpolation functions that are well known by those skilledin the industry. For gas temperatures less than rated, the equation forfield current is given by

IF@TLOW=SQRT((IF_RAT{circumflex over ( )}2+TMPSFB*(TGAS-TGAS_RATED))  (2)

[0026] For gas temperatures greater than rated, a differentinterpolation function is more appropriate and is given by

IF@THIGH=SQRT((IF_RAT{circumflex over ( )}2+TMPSFA*(TGAS-TGAS_RATED))  (3)

[0027] The coefficients TMPSFA and TMPSFB can be determined, usingEquation (2) or (3), from generator data supplied by the generatormanufacturer relating field current at a given coolant temperature tofield current at rated temperature. Similar to hydrogen pressurecompensation, both sides of Equation (2) and (3) can be divided byIF_RAT to yield a multiplier that can be used to modify feedback signalsand/or modify limit setpoints so that fixed protection and limiteralgorithms can be used. In the preferred implementation this multiplieris again called HTS_BIAS and is scaled, in multiplier block 575, 10000counts for unity ratio. The preferred implementation automaticallyselects between hydrogen pressure or temperature compensation based onthe entry of rated temperature (TMPRAT). That is, a value of 0 forTMPRAT results in selecting hydrogen pressure compensation while anon-zero value for TMPRAT selects temperature compensation, via switch570. As with hydrogen pressure, the preferred implementation utilizes anoffset current signal so that an open wire condition can be easilydetected. In the case of temperature, a non-zero input is required forTMPRAT so that a signal WRKI4, which represents the difference betweenthe actual temperature and rated temperature, can be calculated bysumming block 540. Because TMPRAT has a value other than zero, the lowerpath, namely, function block 555, on the upper right hand side of FIG. 5is selected, via switch 570. This results in a correct implementation ofEquation (2) or (3) when both sides of Equation (2) or (3) are dividedby IF_RAT with IF_RAT replaced by a count value of 5000 counts. 5000counts is used because equipment is typically scaled for 5000 counts atAFFL (where AFFL is the field current required at full load, rated powerfactor and at rated temperature). This count value, however, isarbitrary and can be changed depending on implementation circumstances.The preferred technique is to use a ratio block to multiply the feedbacksignal to the inverse time protection algorithm by 10000/HTS_BIAS andthe reference to the OEL (FCR_REF) by HTS_BIAS/10000 as shown in FIG. 4,blocks 435, 430. That is, the feedback signal is intercepted andmodified in the desired manner.

[0028] The Under Excited Limit (UEL) preferably should account for endiron heating, stator lamination over voltage and steady state stabilityconcerns in arriving at a setting which will adequately protect thegenerator and maintain system stability.

[0029]FIG. 1 illustrates a typical generator capability curve for anair-cooled combustion turbine driven generator. In a preferredembodiment there is provided a method and apparatus for modifying theUEL in the area of the generator capability curve limited by end ironheating and stator lamination over voltage effects by modifying afunction generator. The function generator is used to approximate theunder excited capability of the generator (shown as curve CD in FIG. 1).The generator underexcited capability at zero real power is a singlevalue and then varies at other power levels based on either coolanttemperature or coolant pressure. A preferred UEL coolant compensationimplementation is illustrated in FIG. 6. The function generator outputis shown as UEH@IN in FIG. 6 and is used as an input to summing block650 of UELHT block 410. The preferred implementation then modifies thefunction generator output by a function that varies with the coolantcondition (5000-HTSMAXVA)*(UEHTC1 or UEHTC2) (where HTSMAXVA varies withcoolant condition) times either the real power or real power squared, asdepicted by function blocks 615, 620, 625, 630, 670 and 675. Generally,best results are obtained using real power for hydrogen-cooledgenerators and real power squared for air-cooled generators. Jumper orswitch block 695 is used to select either real power with a setting of 0or real power squared with a setting of 1. UEHTC1 is a coolantcoefficient used to modify the function generator output as a functionof hydrogen pressure or ambient temperature (for ambient temperaturegreater than rated) and UEHTC2 is a coolant coefficient used to modifythe function generator output for ambient temperature less than rated.From the generator capability curves illustrated in FIG. 1, the functiongenerator, block 405, is selected to approximate the curve DCL where thecurve from C to L is just an extension of the rated temperature curvefrom rated KVA at 0.95 power factor to a real power point equal to 120%of the rated KVA. For the case shown in FIG. 1, margin e.g. 10%, isadded to the capability curve using UEHTC4 and results in the dashedcurve MNP. To complete the desired UEL characteristic, a straight-lineis added through points N and Q and a “max value” selection is madeusing block 660 of FIG. 6. The slope of the line through N and Q in FIG.1 is calculated as (sin (arccos (0.95)-margin))/(1-0.95). UEHTC3 is thenfound by multiplying the slope of the line through N and Q by 1000. Forthe capability curves in FIG. 1, UEHJMP.0, block 695, is set to a 1 fora second order interpolation and UEHTC1 is found using the followingformula

UEHTC1=5000*VAR1*GEN_MVA{circumflex over ( )}2/(WATT1{circumflex over ()}2*(GEN_MVA-MVA@WATT1))   (4)

[0030] Where VAR1 is the difference in MVARS between the hightemperature generator capability curve at point G and the ratedtemperature capability curve DCL, GEN_MVA is the rating of the generatorat rated temperature, WATT1 is the rating in MWATTS at point G, andMVA@WATT1 is the MVA rating of the generator at point G. Similarly,UEHTC2 is found using data from the low temperature capability curve DKand is given by the equation

UEHTC2=5000*-VAR2*GEN_MVA{circumflex over ( )}2/(WATT2{circumflex over ()}2*(GEN_MVA-MVA@WATT2))   (4)

[0031] Where VAR2 is the difference in MVARS between point K and therated generator capability curve DCL, WATT2 is the MWATT rating of pointK and MVA@WATT2 is the MVAR rating of point K.

[0032] For the case of a hydrogen-cooled generator, UEHJMP.0, block 695,is set equal to 0 for a linear interpolation with power and UEHTC1 isgiven by the equation

UEHTC1= 5000*(VAR1*GEN_MVA)/(WATT1*(GEN_MVA-MAXVA))   (5)

[0033] Where VAR1 is the difference in MVARS between the curve at lowesthydrogen pressure and most leading power factor for which constantstator MVA is maintained and the curve at rated hydrogen pressure. WATT1is the rating in MWATTS of the generator at the lowest hydrogen pressureand most leading power factor for which constant stator MVA ismaintained. MAXVA is the MVA rating of the generator at the lowesthydrogen pressure. Because hydrogen pressure is assumed to be at ratedor less, UEHTC2, block 625, is entered as zero. The margin is entered inUEHTC4, block 655, and the slope in UEHTC3, block 685, as was done forthe air-cooled generator.

[0034]FIG. 5 illustrates the preferred method for obtaining a value forthe term HTSMAXVA. The generator KVA varies as a logarithmic function ofabsolute hydrogen pressure for hydrogen cooled generators, but isdefined by a quadratic interpolation function for air cooled generators.This is similar to the variation of field current with coolantconditions, but different exponential coefficients for hydrogen cooledgenerators and quadratic interpolation functions for air cooledgenerators are appropriate. These functions are shown in the bottom halfon the right side of FIG. 5 and result in the generation of signalHTSMAXVA that is scaled for 5000 counts in block 585 at a ratio ofMAXKVA/GEN_KVA of unity. As in the case of field current, the preferredembodiment of HTSMAXVA automatically selects hydrogen pressure if TMPRATis entered as zero, block 580. The value to use for air-cooledgenerators is selected based on whether temperature is greater than orequal to rated or less than rated, blocks 560, 565 and 580.

[0035] To properly normalize HTSMAXVA to account for variations ingenerator terminal voltage requires that HTSMAXVA be divided by per unitgenerator voltage squared. This is accomplished in FIG. 5 using blocks505, 510, 535, 536, 537, 538, 539, and 541. Blocks 537, 538, 539 and 541comprise a digital low pass filter with a filter time constant, UELVLPentered in block 538. Filtered generator voltage is used to remove anyinteractions with local generator dynamics. Ratio blocks 505 and 510 areused to divide the output of block 585 by per unit generator voltage.One of the inputs to multiplier block 505 is 20000 resulting in amultiplication of the signal from block 585 by 20000. Block 510 is aratio block that divides the input at x, 20000 times the output fromblock 585, by the input at y, filtered generator voltage. The value20000 is used in this particular implementation for rated generatorvoltage. Any suitable value could be used. Blocks 535 and 536 thendivide block 585-output/per-unit generator voltage by per unit generatorvoltage resulting in the desired signal at the output of block 536 beingproperly compensated by (1/(per unit generator voltage squared)).

[0036] Typically, generators have the capability of operatingunderexcited at rated KVA to some leading power factor, which may or maynot be the same as rated lagging power factor. This is illustrated bypoint C on FIG. 1. When this power factor point is reached there is adiscontinuity in the curve that determines the under excited limit. Theunder excited limit for real power levels that exceed the discontinuityis determined by the constant KVA or stator current curve which is astraight line with constant slope through points N and Q of FIG. 1. Ifone were to try to modify the output of the function generator UEH@IN ofFIG. 4 to account for the discontinuity, it would be difficult to obtaina single compensation term because the slopes are very different in thetwo areas, as can be readily seen in FIG. 1. In the preferredimplementation a high value gate 660 (FIG. 6) is used to select betweenthe compensated function generator output from summing block 655(discussed below) and the output from the compensated constant KVAfunction, divider block 690. While it would be possible to generate theconstant KVA function exactly, the preferred implementation utilizes astraight-line approximation to the constant KVA function. This is doneto eliminate the possibility of infinite (or very large) slope (deltaVars/delta Watts) in the constant KVA function resulting in infinite orvery high gain which would adversely impact the stability of the underexcited limit (UEL). This is accomplished in the preferredimplementation as shown in FIG. 6.

[0037] Specifically, a signal proportional to the maximum KVA at a givencoolant condition (HTSMAXVA) is subtracted, in summing block 680 fromthe real power signal (UELWATTS). The desired slope (UEHTC3/1000) thenmultiplies this difference, via blocks 685 and 690. The result is astraight line limiter characteristic that varies with generator coolantconditions having a fixed slope that provides limiter operation in thegenerator constant KVA section of the generator capability curve (curveNQ). Using a fixed slope avoids the potential for instability caused byinfinite or very large slope in the delta Vars/delta Watts'scharacteristic if the actual constant KVA curve were implemented.UEHTC4, input to summing block 655, provides for margin to be added tothe function generator part of the underexcited capability curve. Thispermits the user to enter points from the actual capability curve (DC ofFIG. 1) and then include the margin as a separate input. Thestraight-line limit passes through points N&Q.

[0038]FIG. 2 illustrates a typical generator capability curve for ahydrogen-cooled generator. The rated capability curve for ahydrogen-cooled generator is shown as curve ABCD in FIG. 2 and occurs atthe maximum generator hydrogen pressure. Ratings at other than ratedpressure result in less generator capability. As in the case ofair-cooled generators, curve DCL is the underexcited capability of thegenerator and is set using the arbitrary function generator, block 405,in FIG. 4. Margin is added to the curve using UEHTC4, block 655 of FIG.6, resulting in curve MNP in FIG. 2. For hydrogen cooled generators,UEHJMP.0, block 695, is set to a zero resulting in a linearinterpolation with UELWATTS. UEHTC1, block 620, is set to

UEHTC1=5000*VAR1*GEN_MVA/(WATT1*(GEN_MVA-MAX_MVA)   (6)

[0039] Where VAR1 and WATT1 are shown on FIG. 2 and MAX_MVA is the MVArating of the generator at point G of FIG. 2. Because hydrogen pressureis assumed to always decrease from rated, UEHTC2, block 625, is set tozero. As with the air-cooled generator, the UEL characteristic isdetermined by a maximum value select circuit, block 660 in FIG. 6,between the curve MGP and a straight line segment through NQ of FIG. 2.Blocks 680, 685, and 690 of FIG. 6 are used to generate a straight-linesegment with a slope determined by UEHTC3, block 685. Block 690 is usedto normalize the slope, 1000 counts equals unity slope. UEHTC5, block665, is used to limit the limit the UEL reference between −32767 countsand UEHTC5 counts.

[0040] Utilizing a two-part characteristic for the UEL curve providesthe ability to approximate the generator under excited capability curvewhen there is no variation with coolant conditions for generator outputlevels less than rated KVA. This is a common practice, particularly withEuropean generator manufacturers, and results in a single underexcitedcapability curve up to the constant KVA portion of the capability curve.This is shown as FIG. 3. At generator KVA output levels less than ratedfor the given coolant condition, the UEL characteristic is determined bythe function generator output, 600 in FIG. 6. The function generatoroutput will not be modified as a function of coolant if coolantcoefficients UEHTC1, block 620, and UEHTC2, block 625, are set equal tozero. All the variation with coolant condition occurs in the constantKVA region and can be accommodated by selecting the proper slope usingUEHTC3, block 685. The middle temperature curve is selected as the ratedtemperature curve. At point C of FIG. 2, add the desired margin to theMVAR rating in per unit (sin (arccos (0.95)-0.1). The slope from FIG. 2is then found from

Slope=(sin(arccos(0.95))−0.1)/(1.24−(0.95*1.24)))   (7)

[0041] UEHTC3, block 685, is found by multiplying the slope by 1000 andUEHTC4, block 655, is found by multiplying the margin in per unit by5000. Curve MNP of FIG. 3 illustrates the capability of the generator inthe underexcited region with the appropriate margin added by UEHTC4. Thestraight line through points NQ illustrates the capability in theconstant stator MVA part of the capability curve. The slope of the lineis determined by UEHTC3, block 685, and normalized by block 690.Variation with ambient temperature is accomplished by varying HTSMAXVA,block 565 of FIG. 5. The UEL characteristic is determined by a maxselection of the function generator with margin, block 655, and thecompensated straight line slope, block 690, using a max select function,block 660.

[0042] In the overexcited part of the capability curve (curves AB, EF,and HJ of FIG. 1), field heating is the limiting parameter and atechnique has been described for implementing an over excitation limiter(OEL) which can be compensated for coolant conditions to enable theexciter to adequately protect the generator field and yet providetransient forcing capability for system events. In the underexcited partof the capability curve, it has been shown that the under excited limit(UEL) can be compensated for variations in coolant conditions using amax value selection, via block 660 in FIG. 6, of a two partcharacteristic. A function generator (FIG. 4) which can be modified toreflect coolant conditions is used to implement the limit characteristicin the area of the capability curve less than rated generator KVA at theparticular coolant condition. A straight-line limit in the constant KVAsection can be compensated for coolant conditions and has been shown toprovide a very good approximation to the underexcited capability curvein the area limited by rated KVA at the given coolant condition. Thestraight-line approximation decreases the potential for instability inthe UEL caused by the very high slope of the actual capability curve,especially around unity power factor. The area of the capability curvenot covered by either the OEL or UEL is from rated KVA at rated laggingpower factor to rated KVA at unity power factor (curve BQ in FIG. 1).This is the area of the generator capability curve that is limited bythe turbine output and by stator current heating. The exciter can onlytransiently affect real power requiring the exciter to reduce generatorreactive output power as a means of reducing generator output KVA orgenerator stator current. Thus, there is needed a stator currentlimiter, which operates to change the setpoint of an associatedautomatic voltage regulator (AVR) (not shown) to reduce reactive power.In turbine ambient following applications it is desirable in accordancewith the preferred embodiment to be able to bias the stator currentlimiter so that the limit changes as a function of coolant conditions.This is easily accomplished in the preferred implementation because asignal already exists which can be used to modify the limiter to matchthe change in generator capability as coolant is changed.

[0043] Specifically, HTSMAXVA is a signal that is scaled for 5000 countsat rated coolant conditions and is compensated for changes in generatorterminal voltage. As coolant conditions change, this signal changes toreflect the change in generator capability. For instance, a decrease incold gas temperature from rated on an air cooled generator will resultin HTSMAXVA increasing based on the constant TMPSSB which is selectedbased on generator ratings for temperatures less than rated. TMPSSB isgiven by the equation

TMPSSB=250,000*((MAXVA@T_LOW/GEN_MVA){circumflex over ()}2-1)/(T_RAT-T_LOW)   (8)

[0044] Where MAXVA @T_LOW is the generator rating in MVA at the lowtemperature rating of the generator, T_RAT is the rated temperature ofthe generator in degrees Celsius and T_LOW is the low temperature ratingof the generator in degrees Celsius.

[0045] TMPSSA is likewise given by the equation

TMPSSA=250000*(1-(MAXVA@HI_TEMP/GEN_MVA){circumflex over ()}2)/(T_HIGH-T_RATED)   (9)

[0046] Where MAXVA@HI_TEMP is the rating of the generator in MVA at thehighest rated temperature and T_HIGH is the temperature in degreesCelsius at the highest rated temperature.

[0047] Since a stator current limit is implemented with a fixedsetpoint, the stator current feedback is preferably altered in order toprovide compensation for temperature. This is accomplished by using theE1PFV block, block 425 of FIG. 4, which includes the ability to dividethe actual stator current feedback signal by HTSMAXVA/5000.

[0048] The preferred implementation generates an accurate representationof the generator capability for changes in coolant for both air-cooledand hydrogen cooled generators using constants H2SEXP for hydrogencooled generators and TMPSSA and TMPSSB for air cooled generators. Thischange in generator capability can be used to modify the feedbackvariable thus permitting fixed setpoint limiters to be used.

[0049] Thus, in a preferred implementation, in the case of the OEL whichinvolves field current, there is preferably provided a field currentregulator. The OEL becomes the upper part of the generator capabilitycurve (curve AB of FIG. 1). It is desirable that the exciter be able totransiently force more than rated field current through the generatorfield and thus there is provided an inverse time function to approximatethe capability of the generator field to accommodate the transient fieldcurrent. In accordance with the preferred embodiment, there is providedthe ability to change both the limiter and protection algorithms toreflect the fact that the generator field capability changes as afunction of the cooling conditions of the generator. Thus, there iscoordination between the limiter and protection functions even at thesenon-rated coolant conditions.

[0050] The UEL (Under Excited Limiter) performs a similar function butin an area of the generator capability curve that is limited by end ironheating in the generator and steady state stability. This is basicallythe lower part of FIG. 1 (curves DC, DG or DK). The curve shown frompoint D to point C comprises the underexcited capability of thegenerator from zero real power to 0.95 per unit real power (point C is 1per unit KVA at 0.95 power factor in this case). This is an empiricalcurve. The curve shown from points C to B is a circle with center at 0watts and 0 vars and a radius of 1 per unit KVA. This curve isapproximated with the UEL portion of the preferred embodiment using astraight line with fixed slope from point N through point Q. Thestraight line with fixed slope approximation is used rather than theactual circle because the slope of the circle at point Q has infinitegain (delta Q/delta P) which would result in infinite gain in the UELregulator (not shown). A high value select circuit, block 660, is usedto take the higher of the curve shown from D to C or the straight linesegment from N to Q. There is no corresponding protection function forthe UEL that requires coolant compensation but the instantimplementation provides a very good approximation to the variation ofthe generator capability curve with coolant conditions.

[0051] Similar to the UEL, a stator current limit function is used torestrict operation of the generator in the area of the capability curvelimited by generator stator current. This is the constant KVA area shownas the right hand part of the capability curve of FIG. 1 (curve BC) andis the same circle used by the UEL. In this case, a circlecharacteristic is used because the stator current limit is a set point,very low bandwidth, controller. The disclosed algorithms provide veryaccurate implementation of the circular generator capability curves as afunction of generator cooling conditions.

[0052] While the foregoing description includes numerous details andspecifics, it is to be understood that these are provided for purposesof explanation only, and are not intended to limit the scope of theinvention. Those of ordinary skill in the art will easily be able tomake numerous modifications to the exemplary embodiments described abovewithout departing from the scope of the invention, as defined by thefollowing claims and their legal equivalents.

What is claimed is:
 1. A method for providing protection to a generator,comprising the steps of: intercepting a feedback signal of a functiongenerator indicative of rated capability; determining, based on acoolant condition of said generator, a compensation value in accordancewith a change in generator performance due to said coolant condition;and modifying said feedback signal, based on said compensation value, toaccount for said change in generator performance.
 2. The method of claim1 , further comprising multiplying said feedback signal by saidcompensation value.
 3. The method of claim 1 , further comprisingcompensating at least one of an over excited limit (OEL) and an underexcited limit (UEL) reflecting limits of operation based on a capabilitycurve of said generator.
 4. The method of claim 1 , further comprisingcompensating at least one of a setpoint of an automatic voltageregulator associated with said generator and an inverse time protectionfeedback signal.
 5. The method of claim 1 , further comprising readingan output of one of a temperature sensor and a pressure sensor.
 6. Themethod of claim 1 , further comprising determining whether said outputis above a minimum threshold, and if not, rendering said compensationvalue determining and feedback signal modifying steps ineffectual. 7.The method of claim 1 , wherein said method is operable with at leastone of a steam turbine generator and a gas combustion generator.
 8. Themethod of claim 5 , further comprising scaling said output.
 9. Themethod of claim 5 , further comprising summing said output with a ratingvalue of said generator.
 10. The method of claim 9 , further comprisingraising a result of said summing step to a power representative of alog-log relationship between coolant gas pressure and allowable fieldcurrent in said generator.
 11. The method of claim 9 , furthercomprising employing a result of said summing step in an equationindicative of a quadratic relationship between coolant gas temperatureand allowable field current in said generator.
 12. The method of claim 3, further comprising employing a two-part characteristic to approximatesaid UEL.
 13. A method of controlling an electrical generator,comprising the steps of: determining the rating of said generator;monitoring a cooling condition of said generator; calculating, based onsaid rating and cooling conditions, a compensation value for limits ofpredetermined portions of a capability curve of said generator; andmodifying a feedback signal to a fixed limiter function in accordancewith said compensation value.
 14. The method of claim 13 , furthercomprising monitoring one of a temperature sensor and pressure sensor tomonitor said cooling condition.
 15. The method of claim 14 , furthercomprising low pass filtering an output of said sensor.
 16. The methodof claim 13 , further comprising summing a generator rating value with avalue indicative of said cooling condition and employing the resultingsum as a multiplier to effect said modifying step.
 17. The method ofclaim 13 , further comprising automatically selecting one of effectingsaid calculating step based on generator coolant temperature and coolantpressure.
 18. The method of claim 13 , wherein said calculating stepcomprises a calculation based on a log-log relationship betweengenerator field current and coolant pressure.
 19. The method of claim 13, wherein said calculating step comprises a calculation based on aquadratic relationship between generator field current and coolanttemperature.
 20. The method of claim 13 , further comprising varying afeedback signal to at least one of an underexcited fixed limiterfunction, an overexcited fixed limiter function, an automatic voltageregulator and an inverse time limit and protection function.
 21. Themethod of claim 13 , further comprising scaling a value indicative ofsaid cooling condition.
 22. The method of claim 13 , further comprisingcontrolling the amount of at least one of field current and statorcurrent in said generator.
 23. The method of claim 13 , furthercomprising normalizing with respect to generator terminal voltage saidcompensation value.
 24. The method of claim 13 , further comprisingelectronically emulating at least one portion of said capability curve.25. The method of claim 13 , further comprising providingtransient-forcing capability for said generator by coordinating saidlimiter function with a protection function of said generator.
 26. Anapparatus for providing protection to a generator, comprising: means forintercepting a feedback signal of a function generator indicative ofrated capability; means for determining, based on a coolant condition ofsaid generator and said rated capability, a compensation value inaccordance with a change in generator performance due to said coolantcondition; and means for modifying said feedback signal, based on saidcompensation value, to account for said change in generator performancecapability.
 27. The apparatus of claim 26 , further comprising means formultiplying said feedback signal by said compensation value.
 28. Theapparatus of claim 26 , further comprising means compensating at leastone of an over excited limit (OEL) and an under excited limit (UEL). 29.The apparatus of claim 26 , further comprising means for compensating atleast one of a setpoint of an automatic voltage regulator associatedwith said generator and an inverse time protection feedback signal. 30.The apparatus of claim 26 , further comprising means for reading anoutput of one of a temperature sensor and a pressure sensor.
 31. Theapparatus of claim 30 , further comprising means for determining whethersaid output is above a minimum threshold, and if not, for bypassing saidmeans for determining a compensation value and means for modifying saidfeedback signal.
 32. The apparatus of claim 26 , wherein said apparatusis operable with at least one of a steam turbine generator and a gascombustion generator.
 33. The apparatus of claim 30 , further comprisingmeans for scaling said output.
 34. The apparatus of claim 30 , furthercomprising means for summing said output with a rating value of saidgenerator.
 35. The apparatus of claim 34 , further comprising means forraising a result of said means for summing to a power representative ofa log-log relationship between coolant gas pressure and allowable fieldcurrent in said generator.
 36. The apparatus of claim 34 , furthercomprising means for employing a result of said means for summing in anequation indicative of a quadratic relationship between coolant gastemperature and allowable field current in said generator.
 37. Theapparatus of claim 28 , further comprising means for employing atwo-part characteristic to approximate said UEL.
 38. An apparatus forcontrolling an electrical generator, comprising: a summing block havingas inputs a rating value of said generator and a value indicative of acooling condition of said generator; a circuit for calculating, based onan output of said summing block, a compensation value for limits ofpredetermined portions of a capability curve of said generator; and atleast one of a ratio block and a setpoint block receiving saidcompensation value and outputting a feedback signal to a fixed limiterfunction of said generator.
 39. The apparatus of claim 38 , furthercomprising at least one of a temperature sensor and pressure sensor formonitoring said cooling condition.
 40. The apparatus of claim 39 ,further comprising a low pass filtering connected between said sensorand said summing block.
 41. The apparatus of claim 38 , furthercomprising a switch for automatically selecting one of effectingcalculations based on generator coolant temperature and coolantpressure.
 42. The apparatus of claim 38 , wherein said circuit comprisesmeans for calculating a result based on a log-log relationship betweengenerator field current and coolant pressure.
 43. The apparatus of claim38 , wherein said circuit comprises a means for calculating a resultbased on a quadratic relationship between generator field current andcoolant temperature.
 44. The apparatus of claim 38 , further comprisingconnection to inputs of at least one of an underexcited fixed limiterfunction, an overexcited fixed limiter function, an automatic voltageregulator and an inverse time limit and protection function.
 45. Theapparatus of claim 38 , further comprising a scaling block connectedupstream from summing block.
 46. The apparatus of claim 38 , whereinsaid apparatus controls the amount of at least one of field current andstator current in said generator.
 47. The apparatus of claim 38 ,further comprising a normalizing circuit for normalizing saidcompensation value with respect to generator terminal voltage.
 48. Theapparatus of claim 38 , further comprising a lookup table for emulatingat least one portion of said capability curve.
 49. The apparatus ofclaim 38 , wherein the apparatus comprises a part of an exciter of saidgenerator.